1. Field of the Invention
The present invention relates to methods and apparatus for tuning a traveling wave tube of the coupled cavity type. More particularly, this invention pertains to a traveling wave tube that includes loss buttons of improved design.
2. Description of the Prior Art
The traveling wave tube is a type of vacuum device which serves as an amplifier of microwave frequency energy. It relies upon the electromagnetic interaction that can occur between an electron beam and a microwave frequency signal. The microwave signal propagates along a slow wave structure that causes it to traverse an extended distance between two axially spaced points. This reduces the effective lateral propagation velocity from that of light to that of the electron beam velocity and transfers energy from the beam to the signal. By lowering the propagation velocity, an energy transferring electronic interaction coupling caused to take place between the beam and the microwave signal that amplifies the microwave frequency energy.
The conventional coupled cavity type traveling wave tube comprises an arrangement of interconnected cells that are serially disposed and adjacent one another along a common axis. A plurality of axially aligned passages through the cavities permits passage of the beam and each interaction cavity is coupled to an adjacent cavity by means of a coupling aperture in an endwall. Conventionally, the coupling apertures between adjacent cavities are alternately disposed on opposite sides of the electron beam axis. An electron gun containing a cathode is located within the tube for furnishing a source of electrons that are formed into a beam and directed along a straight path through the cavity passages. The electromagnetic interaction occurs along the electron beam and the microwave signal appearing at the cavity proximate the beam.
The beam is confined or focussed to the axial path by magnetic means to minimize spreading. So-called pole pieces define the cavities and walls of the slow wave structure while magnets positioned outside the vacuum region of the tube provide the magnetic flux. Protruding ferrules project from the front and back sides of the pole piece walls, serving to surround the electron beam passage and providing a concentrated, axially-extending magnetic field between the ferrule of one pole piece and that of an adjacent pole piece. The beam passage formed in the pole piece between the ends of the ferrules functions as a drift tube region.
A common tube structure also includes one or more termination pieces for absorbing spurious microwave signal energy. Such termination pieces, formed of an appropriate ceramic material such as aluminum oxide or beryllium oxide impregnated with silicon carbide eliminate undesired signal reflection in the tube that can result from passive devices coupled to the input and output ends thereof. Such element(s) are located within a termination cavity that can include metallic elements such as sever and termination pole pieces. Depending upon the type of tube employed, the pole pieces may be of either iron (magnetic) or copper (non-magnetic) composition in accordance with the chosen mechanism for focussing the electron beam.
While the termination piece(s) serves as a circuit element for obtaining broadband power absorption, traveling wave tubes also commonly employ so-called resonant loss buttons that are tuned to attenuate specific frequencies. Such buttons are designed to provide a predetermined amount of insertion loss over a specified frequency range. The absorption of such power will reduce the gain of the tube at such frequencies, preventing oscillation and eliminating undesired outputs. The loss buttons are pill-shaped elements formed of an appropriate ceramic material such as magnesium oxide and silicon carbide (MgO Sic) and are designed to fit into circular aperture-like cavities formed in the cavity spacers.
The frequency of attenuation achieved by a loss button is a function of a number of factors including the material composition of the button and the air gap formed between the planar surfaces of the button and the pole pieces. The axial thickness of the button is reduced by grinding after delivery from a vendor to increase the resonant frequency and thus the ceramic buttons are commonly fabricated to a slightly thicker or lower frequency dimension. (Note: The grind is conical or semispherical and does not affect the overall dimension of the button.)
The manufacture of the traveling wave tube requires a brazing step to assure the ability of the tube to hold a vacuum. The heating and accompanying flow of alloys that occurs during brazing can adversely affect the functioning of the loss buttons by changing the air gap described above. Further, the absorption of r.f. power during operation can also produce heating that reduces the air gap to lower the resonant frequency of the loss button cavity possibly to frequency bands of very high power content. This can result in the destruction of the button(s) and the consequent degradation of tube performance..